Bolometers are a standard magnetic fusion plasma diagnostic. However, it is problematic to have a sufficient number of channels and views to accurately reconstruct the plasma radiation profile. In addition, every discrete detector channel generally has 2-4 wire leads for carrying low-level signals away from the plasma and through a vacuum interface, typically resulting in complicated wiring harnesses inside high-temperature, high-nuclear-radiation environment devices, and leading to high cost and complexity.
Current bolometry for fusion plasmas employs discrete thin-foils which are heated by plasma radiation, and the resulting temperature rise is detected by a metal resistor or thermistor bonded to the back side of the foil, often separated by an insulating film. Occasionally, a single-channel infrared (IR) detector has been used to monitor the rise in temperature of the foil, so as to provide better electrical noise immunity. In the case of short-pulse plasma radiation, or when studying the energy deposited by ion, neutral, or electron beam, IR imaging of the back side of a foil or plate target has been employed for determining the instantaneous distribution of energy in the beam. The difficulty with using this technique to observe a long-pulse plasma, is that the lateral heat flow in the foil or plate interferes with subsequent images and confuses the measurement. In addition, a foil which is sufficiently thin to have reasonable time response must be cooled for longer time intervals in order to prevent melting or radiation damage, or nonlinear effects from adversely affecting the measurement. Finally, the detectors must be radiation hardened in order to survive the neutron and gamma fluences from a long-pulse DD or DT machine. So-called "silicon bolometers" and pyroelectric detectors have the advantage of responding directly to the incident power; however, such detectors will not work in these harsh environments.
Plasmas often have complex geometry, and multichannel imaging (as well as tomography) is desired for modeling analysis. A radiation-hard imaging bolometer, using a segmented-absorber matrix, combined with a pinhole-camera geometry and sensitive IR camera has recently been proposed. Such an apparatus would generate several thousand channels of data, while eliminating wires crossing vacuum interfaces.
In "A Rad-Hard, Steady-State, Digital Imaging Bolometer System For ITER," by G. A. Wurden, in Diagnostics for Experimental Thermonuclear Fusion Reactors, edited by P. E. Stott et al. (Plenum Press, New York, March, 1996), pp. 603-606, a pinhole imaging design, with the plasma radiation striking a compact, segmented, rear-surface-cooled "foil," where each segment of the "foil" corresponds to one imaging resolution element or "pixel," is described. Each pixel absorber is raised from the back cooling block (which may be held at a constant reference temperature), and the height of the pixels (and the material thermal conductivity) is adjusted for rapid axial heat flow compared to the relatively longer lateral heat flow path between pixels. The passive absorber matrix is imaged in the infrared, using two metal mirrors, which allows a neutron-sensitive, state-of-the-art, 12-bit digital video IR camera to be positioned out of the line-of-sight of the neutron flux from the plasma. Some of the pixels may be positioned outside of the plasma field-of-view, to act as "background" pixels, if required. The front surface of the absorbing matrix may initially be a "blackened coating" (in accordance with common bolometer design). However, during use, plasma contamination of the surface occurs. No wires exit the vacuum interface. A schematic representation of the bolometer apparatus is shown in FIG. 2 of Wurden. Design parameters are given for an aluminum absorbing matrix having aluminum pixels, although different absorbing materials may be bonded to the end of each pixel according to the teachings of Wurden.
Wurden identifies a potential difficulty in the use of the apparatus as being due to infrared radiation from hot objects (500.degree.-1000.degree. C. hotter than the segmented matrix in the bolometer itself) directly in the field of view. These objects will radiate approximately 1000 times more strongly than the thermal emission from the matrix in the 3-5 .mu.m region, due to blackbody emission. This stray light interferes with the light viewed from the front surface of the segmented matrix. Two suggestions are made for overcoming this difficulty. First, the matrix itself could be operated at elevated temperatures. Additionally, the matrix could be viewed from its back surface, while cooling is applied to the sides of each pixel. No teachings are provided as to how either of these suggestions might be accomplished.
Accordingly, it is an object of the present invention to provide a bolometer for infrared spatial imaging of plasmas which is unaffected by stray light emission from hot surfaces surrounding the plasma.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.